Peak detectors are used in various applications to provide an output voltage equal to or representative of the peak voltage of an input signal. These peak detectors are used in such applications as in telecommunications in which a digital signal is received on a single twisted wire pair without an external clock. The data must be extracted from the signal. A common method used in the extraction of the data from this signal is to detect the peak value of the incoming signal and to sample the incoming signal at a point in the mid range of the incoming signal.
Prior art peak detector circuits generally use an operational amplifier in which the input signal is coupled to one input of the op amp and the output of the op amp is connected through a diode to a capacitor, the capacitor voltage being coupled back to the second input of the op amp. In this circuit the diode is conducting whenever the input voltage is greater than the capacitor voltage and reverse biased otherwise. Thus, when the input signal is rising, the capacitor voltage tracks the input voltage, and when the input signal is falling from a prior peak value, the capacitor voltage is decreasing due to leakage through the diode and other circuitry attached to the capacitor.
However, this prior art circuit presents several problems and difficulties, especially in telecommunications applications described above. If the input signal is noisy, then the prior art peak detector will tend to track the noise peaks on the input signal voltage. Also, this prior art peak detector does not respond rapidly to falling input signals. Third, an operational amplifier for use in the prior art peak detector is difficult to design; it must respond rapidly and yet be stable for the differing load conditions caused by the forward and reverse bias of the diode and the varying amount of charge on the capacitor.
Therefore, it can be appreciated that a peak detector which provides a degree of immunity to noise spikes on the input signal, which has a controlled slew rate, and which avoids the difficulties of designing a stable operational amplifier is highly desirable.